Structure having holes and method for producing the same

ABSTRACT

A structure having a hole, including a substrate, a first layer including an alumina hole, and a second layer disposed between the substrate and the fist layer, wherein the second layer contains silicon, and has a smaller hole than the alumina hole.

This application is a division of application Ser. No. 10/385,570, filedMar. 12, 2003, now U.S. Pat. No. 6,972,146, issued on Dec. 6, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a structure having holes, and a methodfor producing the same. In particular, a nano structure made by using amethod for anodic oxidizing Al according to the present invention isuseful for a wide variety of applications including functional materialsand structural materials for electron devices, memory media and memoryelements. Specifically, the nano structure is effective for use invertical magnetic recording media, patterned media, solid magneticmemories, magnetic sensors, and photonic devices.

2. Description of the Related Art

Some thin films, fine lines, and dots of metals and semiconductors thatare smaller size than a characteristic length may show specificelectric, optical and chemical properties by inhibiting movements ofelectrons. From such a viewpoint, interests on a material having amicrostructure of below 100 nano meter (nm), i.e., nano structure, havebeen increased.

A method for producing the nano structure include semiconductorprocessing technologies including a fine pattern drawing technology suchas photolithography, electron beam lithography, X-ray lithography andthe like.

Other than such production method, there is one approach to provide anovel nano structure based on a naturally formed regular structure,i.e., a self-organized structure. This approach can produce a finerspecial construction than the conventional one depending on a finenessof the structure used as a base, and therefore many studies have beenmade.

Examples of the specific self-organized structure include an anodicallyoxidized alumina film (for example, see C. Furneaux, W. R. Rigby & A. P.Davidson “NATURE” vol. 337, P147 (1989)). An Al plate is anodicallyoxidized in an acid electrolyte to form a porous oxidized film. Thisporous oxidized film is characterized by a special geometric structurewhere highly fine columnar nano holes (microholes) 14 each having adiameter (2 r) of several nm to several hundreds nm are arranged inparallel at a space (2R) of several tens of nm to several hundreds nm asshown in FIG. 2A. The columnar nano hole 14 has a high aspect ratio, andhas excellent uniformity in a diameter profile. The diameter 2 r and thespace 2R of the nano hole 14 can be controlled to some degree byadjusting a current and a voltage upon the anodic oxidation. Ananodically oxidized alumina film is produced on an Al plate 21 via abarrier layer 22.

A wide variety of applications have been tried by focusing on thespecial geometric structure of the anodically oxidized alumina nanohole. The details are described by Masuda. For example, the anodicallyoxidized film is used as a coat utilizing its abrasion resistance andgood insulation, or used as a filter by peeling the coat. Furthermore,various applications includes coloring, a magnetic recording medium, anEL light emitting element, an electrochromic element, an opticalelement, a solar battery and a gas sensor using a technique of filling ametal or a semiconductor into the nano hole, and a technique ofreplicating the nano hole. In addition, a wide range of applicationsincluding quantum effect device such as quantum fine line and MIMelement, and a molecular sensor using the nano hole as a chemicalreaction site are expected (Masuda, Solid State Physics, 31, 493(1996)).

The above-mentioned method for producing the nano structure by thesemiconductor processing technologies has problems of a poor yield andexpensive apparatus. There is a need to provide a simple method forproducing the nano structure with good repeatability.

In view of the above, the self-organization method, especially the Alanodic oxidation method, has an advantage that the nano structure can beproduced easily and controlled well. Typically, these methods canprovide a large area nano structure. However, when an aluminum layer isformed on a substrate, and is anodically oxidized, tightness betweenhole walls and the substrate may be poor.

FIGS. 2 and 3 show a conceptual sectional views of conventional aluminanano holes on an Al plate (film). FIG. 2A is a sectional view showing anAl plate oxidized on an anode. FIG. 2B is a sectional view showing theAl film on the substrate not completely oxidized on an anode. FIG. 3A isa sectional view showing that the anodic oxidation is terminated with abarrier layer remained. FIG. 3B is a sectional view showing that thebarrier layer is removed by dry etching and the like.

The conventional anodically oxidized alumina nano holes are providedonly on a surface of the Al plate (film) as shown in FIGS. 2A and 2B,and their applications and forms are thus limited. For example, Al has amelting point of 660° C., and the nano holes formed on the Al cannot beheated at 660° C. or more. In order to use the nano holes as thefunctional material in various aspects, it is desired to provide atechnology for forming the anodically oxidized alumina nano holes on asubstrate having a high melting point.

In order to apply the anodically oxidized alumina nano holes to anelectronic device and the like, it is desired to provide a technology toembed an enclosing material and to form the enclosing materialconnectable to the under layer. If the anodically oxidized alumina nanoholes can be formed uniformly and stably on the under layer including agood conductive material such as metals, it is possible to form theenclosing material in the anodically oxidized alumina nano holes bycontrolled electrodeposition, whereby the application can be expected tobe broaden.

As an example of forming the anodically oxidized alumina nano holes onthe substrate, Japanese Patent Laid-Open No. 7-272651 discloses atechnology for “forming an Al film on a Si substrate, altering the Alfilm to an anodically oxidized film, removing a barrier layer at abottom of a nano hole part, forming a metal (Au, Pt, Pd, Ni, Ag, Cu)layer capable of forming an eutectic alloy with Si of the Si substrateexposed at the bottom of the nano hole to grow Si needle crystal by aVLS method.”

In the technology, the barrier layer at the bottom of the nano hole isremoved after the Al film is anodically oxidized, in order to penetratethe nano holes to the Si substrate. As the method for removing thebarrier layer, there are cited a method for etching with chromicacid-based etching liquid, and a method for connecting the Si substrateand a counter electrode with an external wire after the anodic oxidationis completed, and for holding the structure in the liquid.

Through intense studies by the present inventors, it is found that afterthe Al film is oxidized on the anode across a total film thickness andthe barrier layer remains as shown in FIG. 3A, it is very difficult tocomplete the anodic oxidation with good repeatability.

Especially when the under layer is disposed under the Al film, and thesubstrate as the under layer or the under layer are made of a lowreactive material, and the anodic oxidation proceeds in the state shownin FIG. 3A, the barrier layer is deteriorated or lost in a very shorttime and the electrolyte is contacted with the substrate (or the underlayer) resulting in an electrolyte decomposition. Even if the anodicoxidation is terminated immediately before the state shown in FIG. 3A,depths of respective nano holes may be deviated to some degree.Accordingly, it is difficult to produce the structure having a uniformbarrier layer remained over a wide range as shown in FIG. 3A.

The structure with the barrier layer remained as shown in FIG. 3A mayrealize in a part of the substrate. In this case, when the barrier layeris then removed, the diameters of the nano holes near the removed partlack linearity and become discontinuous as shown in FIG. 3B, and theshapes of the nano holes are largely different.

In particular, if the nano holes are deep, thicknesses and a proceedingdegree of the anodic oxidation become easily deviated. It is difficultto give the barrier layer with the uniform thickness, and it is almostimpossible to remove the barrier layer by the dry etching and the like.

There is no description about the production of the anodically oxidizedalumina nano holes using noble metal and carbon as the under layer. Itis contemplated that if the under layer is made of these low reactivematerials, water is started to be electrolyzed once the under layer isanodically oxidized and foams are produced, which breaks the anodicallyoxidized film.

One object of the present invention is to provide a structure havingholes penetrating to the predetermined depth area. Other object of thepresent invention is to provide a nano structure having nano holes withexcellent linearity and diameter uniformity where bottoms of the nanoholes are penetrated to an under conductive metal layer, and a methodfor producing a nano structure to form the anodically oxidized aluminanano holes uniformly and stably.

Still other object of the present invention is to provide a nanostructure and a production method therefor providing excellent tightnessbetween an alumina nano hole layer and the substrate, or between thealumina nano hole layer and the under metal layer when the alumina nanohole layer is provided on the substrate via the under metal layer. Thenano structure of the present invention has excellent tightness andtherefore is preferable especially when a polishing step is conductedafter the nano hole production, or when a stress is applied upon the useof the structure.

SUMMARY OF THE INVENTION

According to the present invention, a structure having a hole, includinga substrate, a first layer including an alumina hole, and a second layerdisposed between the substrate and the fist layer, wherein the secondlayer contains silicon, and has a smaller hole than the alumina hole. Athird layer having conductivity may be formed between the substrate andthe second layer. A functional material may be filled into the aluminahole.

A first aspect of the present invention is a nano structure. The nanostructure includes an anodically oxidized alumina nano hole layer on asubstrate formed by an anodic oxidation method, wherein an under layeris formed at a bottom of the anodically oxidized alumina nano hole layervia an adhesive layer having a pore, wherein a nano hole of theanodically oxidized alumina nano hole layer is penetrated to the underlayer through the pore, and wherein the adhesive layer having the porecontains Si as a main component.

In the nano structure, the adhesive layer contains preferably Si and Alas main components, and more preferably is an oxide containing Si andAl. Also, an average diameter (hereinafter also referred to as“diameter”) of the pore in the adhesive layer is preferably not lessthan 1 nm to not more than 9 nm, and the adhesive layer preferably has athickness of 1 to 50 nm.

Depending on the applications, the under layer is preferably conductiveand contains noble metal. Preferably, the nano structure includes anenclosing material embedded into a part or all of the nano hole in theanodically oxidized alumina nano hole layer.

A second aspect of the present invention is a method for producing anano structure having at least an under layer and an anodically oxidizedalumina nano hole layer on a substrate, wherein a nano hole of theanodically oxidized alumina nano hole layer is penetrated to the underlayer, comprising the steps of sequentially laminating an under layer,an AlSi layer for an adhesive layer, and an Al layer on at least asubstrate to form a laminated film, and anodic oxidizing the laminatedfilm to form the anodically oxidized alumina nano hole layer.

In the method for producing the nano structure, the step of forming theAlSi layer for the adhesive layer preferably comprises the step offorming an AlSi layer containing 20 to 70 atomic % of Si using a filmforming technique in which a substance is formed under the conditionthat Al and Si are in a non-equilibrium state.

Preferably, the step of anodic oxidizing is conducted using anelectrolyte containing sulfuric acid. Also, the production methodfurther comprises a step of etching a part of the AlSi layer for theadhesive layer, and the etching step is especially preferably a wetetching step utilizing an acid or an alkali solution. An annealing stepis preferably conducted before or after the etching step.

In the method for producing the nano structure, it is useful forpractical applications to include the step of embedding an enclosingmaterial into the nano hole after the etching step. The enclosingmaterial embedding step is preferably an electroplating step.

Features of the present invention will be described below.

The nano structure of the present invention is a laminated structureincluding a substrate/an under layer/an adhesive layer/an anodicallyoxidized alumina nano hole layer. The anodically oxidized alumina holeis produced by anodic oxidizing a layer containing Al as a maincomponent to oxidize across a total film thickness from a surface to aninterface of the adhesive layer, terminating the anodic oxidation at anadequate time, and then etching it. The pores are formed in the AlSilayer for the adhesive layer after the etching step. The bottom of thenano hole is penetrated to the under layer through the pores of theadhesive layer. The nano hole has good linearity to the interface of theadhesive layer.

The present inventors found that a suitable adhesive layer is disposedat the interface between the anodically oxidized alumina nano hole layerand the under layer at the penetrated part of the bottom of the nanohole, whereby the adhesion strength and the tightness between theanodically oxidized alumina nano hole layer and the under layer areincreased.

In the nano holes according to the present invention shown in FIGS. 1Aand 1B, the nano holes have excellent linearity and diameter uniformity,and the adhesion strength and the tightness between the anodicallyoxidized alumina nano hole layer and the under layer are excellent, ascompared with the conventional nano holes that are subjected to theanodic oxidation step and the barrier layer removing step.

The nano structure of the present invention having excellent adhesionproperties and tightness between the anodically oxidized alumina nanohole layer and the under layer is useful when a stress applying step,i.e., a polishing step is conducted, or a stress is applied upon the useof the structure. Since the nano structure of the present invention hasexcellent adhesion properties, it is relatively stable even if a heattreatment such as an annealing step is conducted. In other words, thenano structure of the present invention has excellent heat resistance,and therefore can be processed at high temperature in the subsequentsteps. The heat treatment can improve chemical stability of theanodically oxidized alumina nano hole.

There are potentials to develop novel electronic devices by embedding ametal, a semiconductor, an oxide and the like into the anodicallyoxidized nano holes of the nano structure according to the presentinvention.

The anodically oxidized nano holes of the nano structure according tothe present invention can be applied to a wide variety of formsincluding a quantum fine lines, a MIM element, an electrochemicalsensor, coloring, a magnetic recording medium, an EL light emittingelement, an electrochromic element, an optical element, an abrasionresistant and insulation resistant coat, and a filter. Thus, itsapplications can be significantly broaden.

Further objects, features and advantages of the present invention willbecome apparent from the following description of the preferredembodiments (with reference to the attached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are conceptual views showing a nano structure accordingto the present invention.

FIGS. 2A and 2B are conceptual views showing conventional anodicallyoxidized alumina nano holes on an Al plate (film).

FIGS. 3A and 3B are conceptual views showing conventional anodicallyoxidized alumina nano holes on an Al plate (film).

FIG. 4 is a process chart showing one embodiment of a method forproducing a nano structure according to the present invention.

FIGS. 5A to 5C are conceptual views showing embedding an enclosingmaterial into anodically oxidized alumina nano holes of a nano structureaccording to the present invention.

FIGS. 6A to 6D are conceptual views showing steps of a method forproducing a nano structure according to the present invention.

FIG. 7 is a schematic view showing an anodic oxidation apparatus.

FIG. 8 is a graph showing a current profile upon anodic oxidation.

FIGS. 9A to 9D are conceptual views showing anodically oxidized aluminanano hole according to the present invention.

FIG. 10 is a schematic view showing an example of a method for formingan AlSi layer for an adhesive layer according to the present invention.

FIG. 11 is a conceptual view showing an AlSi layer for an adhesive layeraccording to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

<Nano Structure Construction>

Referring to Figures, the nano structure of the present invention willbe described.

FIGS. 1A and 1B are conceptual views showing the nano structure of thepresent invention. FIG. 1A shows a plane view, and FIG. 1B shows asectional view along the line A—A. The nano structure shown in FIGS. 1Aand 1B includes a substrate 11, an under layer 12 including a conductivemetal, an adhesive layer 13, nano holes 14, an anodically oxidizedalumina nano hole layer 15 (also referred to as “anodically oxidizedfilm”), and pores 16 disposed in the adhesive layer.

The anodically oxidized film 15 contains Al and oxygen as maincomponents, includes a number of columnar nano holes 14 shown in FIGS.1A and 1B. The nano holes 14 are disposed substantially vertical to asurface of the under layer, and are located in parallel each other atsubstantially equal spaces. Also, the nano holes 14 are prone to bearranged in a triangular lattice as shown in FIG. 1A. Each of the nanoholes has a diameter 2 r of several nm to several hundreds nm, and aspace 2R of about several tens of nm to several hundreds nm.

When the nano holes are arranged in a honey-bomb structure, uniformityof shapes of the nano hole diameters and uniformity of the penetrationto the bottoms of the nano holes are improved. The nano holes can bethus arranged by producing convex and concave portions on the Al surfaceat an adequate space, and by staring the production of the nano holes atthe concave portions. The convex and concave portions are produced byforming concaves on the Al surface or locating a member having convexesand concaves on the aluminum surface.

The spaces and the diameters of the nano holes can be significantlycontrolled by process conditions such as a concentration and atemperature of the electrolyte for use in the anodic oxidation, a methodfor changing an anodic oxidation voltage, a voltage value, a time, andsubsequent etching conditions for widening the pores.

A thickness and a depth of the anodically oxidized alumina nano holelayer can be controlled by a thickness of a film containing Al as a maincomponent, and are 10 nm to 100 μm, for example. Conventionally, thedepth of the nano hole is controlled by the duration of the anodicoxidation. According to the present invention, the depth of the nanohole can be controlled by the thickness of the film containing Al as themain component to provide the anodically oxidized alumina nano holeswith the uniform depth.

The anodically oxidized nano hole layer is preferably made of an oxideof Al, but may contain other elements as long as the nano hole can besuccessfully formed.

The adhesive layer 13 is a film containing Si as a main component withfiner pores than the nano holes. The adhesive layer is obtained from theAlSi layer preferably represented by the formula Al_(1−x)Si_(x) wherex=0.2 to 0.7 before the anodic oxidation. In other words, the AlSi layercontains 20 to 70 atomic % of Si based on the total amounts of Al andSi. After the anodic oxidation or the etching, Al is dissolved, whichleads to a tendency that Si becomes the main component as compared withthe above-defined composition. It is more preferable that the AlSi layercontain 30 to 60 atomic %. Alternatively, a mixture of Si and Ge can beused instead of Si.

The AlSi layer is formed by sputtering Al and Si at the above-definedcomposition ratio. Suitable composition segregation is induced in theAlSi surface, whereby Al columnar structures are formed and dispersed inareas containing Si as the main component. The diameter of the Alcolumnar structure is about several nm. The Al columnar structures aredisposed at a space of about 3 to 10 nm. The diameter and the space ofthe Al columnar structure depend on the film forming conditions and thecomposition ratio of Al and Si. Thus obtained layer is subjected to theanodic oxidation and the etching steps to dissolve the Al columnarstructures, thereby forming the pores 16. When the adhesive layer issubjected to sufficient anodic oxidation, the parts containing Si as themain component are also oxidized to form the adhesive layer containingoxidized Si as the main component. When the adhesive layer is subjectedto suppressed anodic oxidation and etching, the adhesive layercontaining amorphous Si is formed. The pores are preferentially formedin the adhesive layer under the parts where the alumina nano holes arepresent. The thickness of the adhesive layer is not especially limited,but generally 0.3 nm to 100 nm, and more preferably about 1 nm to 50 nm.

When the adhesive layer including the pores contains Si as the maincomponent, it contains 80 to 100 atomic %, preferably 90 to 100 atomic %of Si. The adhesive layer may contains 0 to 20 atomic %, preferably 1 to10 atomic % of Al.

When the adhesive layer including the pores contains Si and Al, itcontains 80 to 100 atomic %, preferably 90 to 100 atomic % of Si basedon the total components other than oxygen. The adhesive layer maycontains 0 to 20 atomic %, preferably 1 to 10 atomic % of Al.

The under layer 12 is not especially limited, but is preferably flat.When the under layer is used as an electrode, it preferably contains aconductive material. Specific examples include noble metals such as Ag,Au, Pt, Pd, Ir, Rh, Os, Ru, and their alloys or Cu, graphite, andsemiconductors such as Si, InP and Ge. The under layer may be a thinfilm or the substrate itself. If the enclosing material is embedded intothe nano holes by electrodeposition, the under layer preferably containsthe noble metal. The nano structure of the present invention has anadvantage that the filling material and the under layer are wellelectrically connected.

Examples of the constructions of the nano structure into which theenclosing material is embedded are shown in FIGS. 5A to 5C. In FIG. 5A,the enclosing material 41 is uniformly embedded into the nano holes upto the surfaces of the nano holes. In FIG. 5B, a laminated film made ofthe enclosing material 42 is embedded into the nano holes. In FIG. 5C,the enclosing material 41 is embedded into the nano holes such that thenano holes are not completely filled. The enclosing material extendingto outside of the nano holes may be embedded into the nano holes (notshown).

If the enclosing material is magnetic, the nano structure may be used asa vertical magnetization film of a magnetic medium, or as a fine line ofa quantum effect device. If Co and Cu are laminated and electrodepositedwithin the nano holes as shown in FIG. 5B, a GMR element in response toa magnetic field can be fabricated. If the enclosing material isembedded into the nanoholes such that the nano holes are not completelyfilled as shown in FIG. 5C, an electron emitting device can befabricated.

If the enclosing material is a light emitting material or a phosphor,the nano structure can be used as a wavelength changing layer as well asthe light emitting device. If a dielectric material other than aluminais embedded into the enclosing material, the nano structure is effectiveas a photonic device.

In the present invention, the enclosing material may not only be presentwithin the anodically oxidized alumina nano holes, but also extend tooutside of the holes. The anodically oxidized alumina nano holestructure according to the present invention can be used as a mask or amold.

<Production of the Nano Structure>

Referring to Figures, a method for producing the nano structureaccording to the present invention will be described. FIG. 4 is aprocess chart showing one embodiment of a method for producing a nanostructure according to the present invention. In FIG. 4, the method forproducing a silicon nano structure of the present invention comprisesthe steps (a) to (d).

Step (a): Film Forming Step

A film forming step comprises the steps (a-1) to (a-3) to from alaminated film including an under layer/an AlSi layer for an adhesivelayer/an Al layer on a substrate.

Step (a-1): The under layer is formed on the substrate. Step (a-2): TheAlSi layer for the adhesive layer is formed on the under layer of thesubstrate using a film forming technique in which a substance is formedunder the condition that Al and Si are in a non-equilibrium state.Thus-obtained AlSi layer for the adhesive layer has columnar structurescontaining Al as a main component, and Si areas surrounding the columnarstructures, and includes a mixed film containing 20 to 70 atomic % ofSi-based on the total amounts of Al and Si.

Step (a-3): Then, an Al film is formed on the AlSi layer for theadhesive layer.

The film forming method in the above-described steps (a-1) to (a-3) maybe any methods including resistance heating vapor deposition, EB vapordeposition, sputtering, and CVD. In any case, the surface of the Al filmis preferably flat.

Step (b): Anodic Oxidation Step

Then, the laminated film obtained in the step (a) is anodically oxidizedto form an anodically oxidized alumina nano hole layer. The Al film isanodically oxidized to form the alumina nano holes. By the anodicoxidation, the Al columnar structures in the AlSi layer for the adhesivelayer are oxidized and dissolved to form the pores. Simultaneously, Siparts of the AlSi layer for the adhesive layer are also oxidized.

Step (c): Etching Step

Non-penetrated parts containing the AlSi layer for the adhesive layer onbottoms of the nano holes, which remain after the anodic oxidation step,are etched to form the pores in the AlSi layer for the adhesive layer.Also, the diameter of the nano hole is broaden.

Step (d): Enclosing Material Embedding Step

After the etching step, the enclosing material is embedded into thealumina nano holes.

Referring to FIGS. 6 to 9, a method for producing the nano structure ofthe present invention will be described. FIGS. 6A to 6D are conceptualviews showing steps of the method for producing the nano structureaccording to the present invention. FIG. 6A is a sectional view showinga film structure before the anodic oxidation. On a substrate 11, anunder layer 12, an AlSi layer for an adhesive layer 31, a filmcontaining Al as a main component 32 are sequentially formed. FIG. 6B isa sectional view showing the film structure after the anodic oxidation.An adhesive layer 13 may have pores 53 or Al columnar structure(s) mayremain in the adhesive layer 13. FIG. 6C is a sectional view showingthat Al parts remained on the adhesive layer are dissolved after theetching, and the diameters of the nano holes are broaden. FIG. 6D is asectional view showing the nano holes filled with the enclosing material41 such as a metal and a semiconductor. FIG. 7 is a schematic viewshowing an anodic oxidation apparatus for use in the present steps.

The steps of FIGS. 6A to 6D will be described for detail. The followingsteps (a) to (d) correspond to the steps of FIGS. 6A to 6D.

(a) Film Forming Step

A sample is produced by forming the under layer 12, the AlSi layer forthe adhesive layer 31 and the Al film 32 on the substrate 11. The filmforming method may be any methods including resistance heating vapordeposition, EB vapor deposition, sputtering, and CVD. In any case, thesurface of the Al film is preferably flat.

According to the present invention, the step of forming the AlSi layerfor the adhesive layer has characteristics, which will be describedbelow.

The AlSi layer for the adhesive layer 31 is formed on the under layer 12of the substrate 11 using the film forming technique in which thesubstance is formed under the condition that Al and Si are in anon-equilibrium state. As the film forming technique in which thesubstance is formed under the non-equilibrium state, the sputtering isused as one example.

On the under layer 12, the AlSi layer for the adhesive layer 31 isformed by a magnetron sputtering method that is the film formingtechnique in which the substance is formed under the non-equilibriumstate. The AlSi layer for the adhesive layer 31 is constituted of Alcolumnar structures 37 containing Al as a main component, and Si areas38 containing Si as a main component therearound as shown in FIG. 11.

Referring to FIG. 10, there will be described a method for forming theAlSi layer for the adhesive layer using a sputtering method as the filmforming method under the non-equilibrium state. FIG. 10 shows asubstrate 1 and a sputtering target 2. With the sputtering method, aconcentration or composition of Al and Si can be easily changed. Asshown in FIG. 10, the AlSi layer for the adhesive layer is formed by themagnetron sputtering method that is the film forming technique in whichthe substance is formed under the non-equilibrium state. The substrate 1is equal to the substrate 11 including the under layer 12.

As shown in FIG. 10, Si and Al sources are fed by disposing Si chips onan Al target (substrate) 2. Although the Si chips 3 are disposed apartin FIG. 10, it is not limited thereto, and single Si chip may be used,as long as the film can be formed as desired. However, in order todisperse the columnar structures containing Al uniformly within the Siareas, the Si chips are preferably disposed symmetry on the substrate 1.

Also, an AlSi sintered product produced by sintering the predeterminedamount of Al powder and Si powder can be used as a target material forthe film forming. Alternatively, the Al target and the Si target areprepared separately, and sputtering may be performed using both targetssimultaneously.

The AlSi layer contains 20 to 70 atomic %, preferably 25 to 65 atomic %,more preferably 30 to 60 atomic % of Si based on the total amounts of Aland Si. When the amount of Si is within the range, there is provided theAlSi layer for the adhesive layer in which the Al columnar structuresare dispersed in the Si areas.

The “atomic %” represents the ratio of Al or Si to the sum of Al and Si,and is also described as atom % or at %. It is obtained by aquantitative analysis of the amounts of Si and Al in the AlSi layer forthe adhesive layer using, for example, an inductively coupled plasmaemission spectrometry.

Although the concentration is herein represented by the atomic %, it canbe represented by wt %. That is, not less than 20 atomic % to 70% orless of Si is equal to not less than 20.65 wt % to 70.84 wt % or less.(Conversion of atomic % to wt % is as follows: A weight ratio of Al toSi is determined using an Al atomic weight of 26.982 and a Si atomicweight of 28.086. A value obtained from (weight ratio)×(atomic %) can beconverted into wt %.)

The substrate has a temperature of 300° C. or less, preferably 200° C.or less. The substrate may have a temperature of not less than 0° C. to100° C. or less, only if the AlSi layer can be formed. In such a way,the AlSi layer for the adhesive layer is formed, resulting in eutecticcrystal morphology where Al and Si are in a metastable state. The Alforms several nm level of nano columnar structures, which are separatedself-organizingly. Such structures are in substantially columnar shapes,and have a diameter of 1 to 10 nm and a space of 3 to 15 nm.

The amount of Si in the AlSi layer for the adhesive layer can becontrolled by, for example, changing the amount of Si chips disposed onthe Al target. When the film is formed under the non-equilibrium state,especially by the sputtering method, a pressure in a reaction vesselwhere argon gas flows is preferably about 0.2 to 1 Pa. The pressure isnot especially limited thereto. Any pressure may be used, as long asargon plasma is formed stably.

The film forming method that the substance is formed under thenon-equilibrium state is preferably the sputtering method, but may beany methods including resistance heating vapor deposition, electron beam(EB) vapor deposition.

As the film forming method, there are a simultaneous process in which Siand Al are formed simultaneously, and a lamination process in which someatomic layers of Si and Al are laminated.

Thus-formed AlSi layer for the adhesive layer 31 includes the Alcolumnar structures 37 containing Al as a main component, and the Siareas 38 containing Si as a main component therearound as shown in FIG.11.

The Al columnar structures 37 containing Al as the main component maycontain other elements such as Si, oxygen and argon as long as thecolumnar microstructures are provided.

The Si areas 38 containing Si as the main component surrounding the Alcolumnar structures may contain other elements such as Al, oxygen andargon as long as the columnar microstructures are provided.

(b) Anodic Oxidation Step

The sample in which the laminated film is formed on the substrate in thefilm forming step is subjected to the anodic oxidation to provide thenano hole structure of the present invention. FIG. 7 is a schematic viewshowing one example of an anodic oxidation apparatus for use in thisstep.

In FIG. 7, the anodic oxidation apparatus includes a constanttemperature bath 60, a reaction vessel 61, a counter electrode 62 suchas a Pt plate, an electrolyte 63, a sample 64, a power source 65 forapplying an anodic oxidation voltage, an ammeter 66 for measuring ananodic oxidation current, and a sample holder 67. The apparatus furtherincludes a computer that automatically control and measure the voltageand current (not shown). The sample 64 and the counter electrode 62 aredisposed in the electrolyte kept at constant temperature by the constanttemperature bath. The power source applies the voltage between thesample and the electrode to conduct the anodic oxidation. The holder 67is for preventing the voltage from applying to undesired parts.

Examples of the electrolyte for use in the anodic oxidation includeoxalic acid, phosphoric acid, sulfuric acid and chromic acid solutions.When the voltage is low (about ˜30 V), the sulfuric acid solution ispreferable. When the voltage is high (60 V ˜), the phosphoric acidsolution is preferable. When the voltage is medium (30 V to 60 V), theoxalic acid solution is preferable. If the Al layer may have a pin holeor pin holes, the electrolyte may be contacted with the under layer toelectrolyze water to induce foams, for example, of oxygen. The foams maybe dispersed by mixing 3% or more of alcohol such as ethanol andisopropyl alcohol into the electrolyte, whereby the anodic oxidation canbe stabilized.

The anodic oxidation will be described. FIG. 8 is a graph showing acurrent profile upon anodic oxidation using various under metal layers.A sample is made by forming the above-described lamination film on thesubstrate made, for example, of quarts. The under layer is used as theelectrode, and the anodic oxidation is performed at a constant voltagein the electrolyte, i.e., the oxalic acid solution. Initially, thesurface of Al is oxidized to rapidly decrease a current value (point Ain FIG. 8). Once the nano holes are started to be formed in the Al film,the current gradually increases and become uniform (point B in FIG. 8).In order to measure an accurate oxidation current, it is required not tocontact the under layer with the electrolyte. When the AlSi layer forthe adhesive layer is subjected to the anodic oxidation (point C in FIG.8), oxidation of Al and diffusion of Al ions to the electrolyte areinhibited to decrease the current value (point D in FIG. 8). Then, theAlSi layer is started to be anodically oxidized (point E in FIG. 8). Atthis point, the anodic oxidation of the top of the Al film is terminatedas shown in FIG. 9B, the Al columnar structures in the AlSi layer forthe adhesive layer are oxidized and dissolved as shown in FIG. 9C, andsimultaneously the Si parts in the AlSi layer for the adhesive layer isoxidized as shown in FIG. 9D. If the anodic oxidation proceeds, thesurface of the under layer may be contacted with the solution toelectrolyze water, thereby increasing the current value (point F in FIG.8). The electrolysis may gradually break the nano holes. If the oxide inthe under metal layer exists stably (Si, Ti, Zr, Hf, Nb, Ta, Mo, W or acombination thereof is mixed therein), the current can be sufficientlydecreased (point G in FIG. 8). The termination point of the anodicoxidation is the point E, or the subsequent point G, or F in FIG. 8.However, it is not preferable that the anodic oxidation is conducted atthe point G or F for a long time, since the under layer is excessivelyoxidized, and the nano holes are broken.

In the anodic oxidation step, the laminated film obtained at the abovestep (a) is anodically oxidized to form the anodically oxidized aluminanano hole layer. The top of the Al film on the laminated film areanodically oxidized to form the alumina nano holes. By the anodicoxidation, the Al columnar structures in the AlSi layer for the adhesivelayer are anodically oxidized and dissolved to form some pores, wherethere remain incomplete pores having non-penetrated part(s).Simultaneously, the Si parts in the AlSi layer for the adhesive layer isoxidized.

(c) Etching

The above-described nano structure is etched, whereby is it possible toremove the non-penetrated part(s) of the bottoms of the nano holes. Theetching may include the step of immersing the structure in an acidsolution, i.e., a phosphoric acid solution, or in an alkali solution,i.e., a KOH solution. The etching can also broaden the diameters of thenano holes. The nano structure having the desired nano hole diameter canbe obtained by controlling an acid concentration, a processing time, atemperature and the like.

(d) Enclosing Material Embedding Step

(Electrodeposition Step)

When the metal is electrodeposited in the nano holes, the substrate isimmersed in a solution containing ionized metal after theabove-described steps, and the voltage is applied to the under layer.One example of the solution is a cobalt sulfate solution. In order tofully produce nuclei upon the electrodeposition, applying voltage AC iseffective. When the metal such as Co, Cu and Ni is electrodeposited, itis required to apply a negative voltage to the under layer, since theseelements discharge cations in the electrodeposition solution.

In the present invention, the formation of the enclosing material byelectrophoresis is also referred to as the electrodeposition. Forexample, since a DNA is negatively charged in the solution, a positivevoltage is applied to the under layer as described above, whereby it ispossible to embed the DNA into the nano holes.

Of course, the enclosing material can be disposed by any film formingmethods such as penetration from the top of the nano holes or a CVDmethod other than the electrodeposition. Also by the electrodeposition,the nano holes can be filled not only with the metal but also with anymaterials such as a semiconductor and an oxide.

In some cases, after the enclosing material is sufficientlyelectrodeposited in the nano holes, it is more effective to polish thesurface of the nano holes in order to be flat.

It is also effective to anneal the nano structure before or after theetching. An annealing temperature is up to 1200° C. Residual water inthe film can be removed by annealing at a temperature of 100° C. ormore. Crystalinity of the anodically oxidized film can be enhanced byannealing at an increased temperature. When the nano structure isannealed after the enclosing material is filled, the properties or thestructure of the enclosing material can be controlled and the tightnesscan be improved. The annealing can be conducted under vacuum, orreducing atmosphere such as hydrogen and inactive gas. As long as theunder layer is not broken, the annealing can be conducted in air or inoxygen.

EXAMPLES

The present invention will be described by the following Examples.

Example 1

This example illustrates the production of penetrated anodicallyoxidized alumina nano holes as shown in FIGS. 6A to 6D.

a) Formation of Under Layer, AlSi Layer for Adhesive Layer and Al Film

On eight quartz substrates, Ti films were formed in a thickness of 5 nmby an RF sputtering method, and then Pt films were formed in a thicknessof 20 nm, respectively. Eight kind of AlSi layers having Al_(1−x)Si_(x)composition containing 10, 20, 30, 40, 50, 60, 70, and 80 atomic %(hereinafter simply referred to as “1%”) of Si, i.e., x=10 to 80%, wereformed thereon. On a top of each AlSi layer for the adhesive layer, anAl film was formed in a thickness of 200 nm.

A target was made of aluminum in a shape of circle with a diameter of100 mm on which 2 to 14 silicon chips in a size of 15 mm square aredisposed. Sputtering conditions were as follows: RF power source, Arflow rate: 50 sccm, discharge pressure: 0.7 Pa, RF power: 1 kW. Atemperature of each substrate was room temperature (25° C.).

In this example, the target was composed of aluminum having 2 to 14silicon chips. The numbers of the silicon chips are not limited thereto,and depend on the sputtering conditions such that the composition of theAlSi layer for the adhesive layer contains silicon in the nearlypredetermined amount. Alternatively, the target may be composed ofsilicon having aluminum chips, or may be composed of a sintered siliconand aluminum.

Thus-obtained aluminum silicon mixture films were observed by a fieldemission scanning electron microscope (FE-SEM). Aluminum columnarstructures were disposed two-dimensionally surrounded by silicon areason the surfaces, viewing from a top of the substrate, as shown in FIG.11. Each of the aluminum columnar structure had a diameter of 1 to 9 nm.The aluminum columnar structures were observed for their sections by theFE-SEM, have a length of 20 nm, and were independent each other.

b) Anodic Oxidation

Using the anodic oxidation apparatus shown in FIG. 7, the anodicoxidation was conducted. In this example, 0.3 mol/L of oxalic acidsolution was used as the electrolyte, and the electrolyte was kept at17° C. with the constant temperature bath. The voltage of the anodicoxidation was DC40V. The under layer of each sample was used as theelectrode so that the anodic oxidation proceeded uniformly. During theanodic oxidation step, the current of the anodic oxidation was monitoredto detect that the anodic oxidation proceeded on the Al surface andreached the under layer. The anodic oxidation was terminated when thecurrent was increased as shown in FIG. 8, point E. After the anodicoxidation, the sample was washed with distilled water and isopropylalcohol.

d) Etching

After the anodic oxidation, each sample was etched by immersing it in a5 wt % phosphoric acid solution for 15 minutes, as needed.

Results

The surface and section of the samples taken were observed by theFE-SEM. As a result, in each sample having the Al_(1−x)Si_(x)composition where x=20 to 70%, the nano holes were penetrated to theunder layer 12 via the adhesive layer as shown in FIG. 1B. Also, in eachsample, the AlSi layer for the adhesive layer oxidized having pores withdiameters of 2 to 8 nm remained between the anodically oxidized aluminanano hole layer and the under layer.

In the samples having the Al_(1−x)Si_(x) composition where x=10% and80%, the shapes of the pores or the penetration of the nano holes werepoor.

The sample of the present invention and a sample including no adhesivelayer were polished with a diamond slurry by a polisher to about half ofthe anodically oxidized alumina nano hole layers. The sample of thepresent invention was not damaged, but the sample including no adhesivelayer was damaged such that a part of the anodically oxidized aluminanano hole layer was peeled. This revealed that the sample of the presentinvention had sufficient adhesion strength. A sample not etched wasproduced to evaluate as described above. In such sample, the nano holeswere penetrated, but some of them were insufficiently penetrated.

Example 2

Samples were prepared using the same procedure described in Example 1,except that each AlSi layer for the adhesive layer had theAl_(1−x)Si_(x) composition where x=40%, the thickness of each AlSi layerwas any of 1 to 100 nm, and the etching was conducted using a KOHsolution for 1 to 10 minutes.

The samples were observed by the FE-SEM. In each sample having theadhesive layer with the thickness of 50 nm or less, the nano holes werepenetrated to the under layer 12 as shown in FIG. 1B. In each samplehaving the adhesive layer with the thickness of more than 50 nm, some ofthe nano holes were not penetrated. In view of the results, it ispreferable that the adhesive layer has a thickness of 1 to 50 nm.

Example 3

Two samples were prepared using the same procedure described in Example1, except that each AlSi layer for the adhesive layer had theAl_(1−x)Si_(x) composition where x=40%, and the anodic oxidation wasterminated at different timings.

Specifically, the anodic oxidation of one sample A was terminated whenan anodizing oxidation current reached the point E shown in FIG. 8, andthe anodic oxidation of the other sample B was terminated after theanodized oxidation current reached the point E shown in FIG. 8. Then,the samples were etched in 5 wt % of phosphoric acid solution for 20minutes.

The samples were observed by the FE-SEM. In the sample A, a Si adhesivelayer 34 having pores penetrated to the under layer 12 was at the bottomof the anodically oxidized nano hole film as shown in FIG. 9C. In thesample B, an oxidized Si adhesive layer 36 having pores penetrated tothe under layer 12 was at the bottom of the anodically oxidized nanohole film as shown in FIG. 9D.

Example 4

Three samples were prepared using the same procedure described inExample 1, except that each AlSi layer for the adhesive layer had theAl_(1−x)Si_(x) composition where x=40%, the thickness of each AlSi layerwas 50 nm, the under layers were SiO₂, Ti, and Pt, the anodic oxidationwas terminated at the point E in FIG. 8, and the etching was conductedusing 5 wt % of phosphoric acid solution for 20 minutes.

The samples were observed by the FE-SEM. In all samples, the Si adhesivelayers having pores penetrated to the under layers 12 were at thebottoms of the anodically oxidized nano hole films as shown in FIG. 9C.

Example 5

Three samples were prepared using the same procedure described inExample 4 such that the under layers were SiO₂, Ti, and Pt, theanodically oxidized alumina nano holes were produced, and an enclosingmaterial was electrodeposited. The electrodeposition was conducted asfollows: the sample was a working electrode, Co was a counter electrode,a plating bath included 5% CoSO₄.7H₂O, 2% H₃BO₃, a voltage was VDC of−2V, and an electrodeposition time was 20 sec.

The electrodeposited samples were observed for their section by theFE-SEM. In the sample including the under layer made of Pt, the sectionhad the construction shown in FIG. 5A. The columnar nano holes having adiameter of about 40 nm were filled with Co, and were arranged inparallel at spaces of about 100 nm each other. The columnar nano holesreached the adhesive layer, and Co was electrodeposited in the pores inthe adhesive layer. However, in the sample including the under layermade of Ti, Co was electroplated only partly. In the sample includingthe under layer made of SiO₂, Co was not electroplated. In view of theresults, the under layer made of noble metal has an advantage in theelectrodeposition step.

Example 6

Three samples were prepared using the same procedure described inExample 3 except that the anodic oxidation was conducted using A: 0.3mol/L of sulfuric acid, at 5° C., 25V, B: 0.3 mol/L of oxalic acid, at15° C., 40V or C, 0.3 mol/L of phosphoric acid, 10° C., 80V.

Among them, the point E in FIG. 8, that was the constant voltage, wasevident when the sulfuric acid A was used. All samples were observed bythe FE-SEM. As a result, the sample that was subjected to the anodicoxidation using the sulfuric acid A had best adhesion between the bottomof the anodically oxidized alumina nano hole layer and the adhesivelayer.

As is apparent from the above-described embodiments, according to thepresent invention, adhesion between the under layer and the anodicallyoxidized alumina nano hole layer is enhanced. Accordingly, theanodically oxidized alumina nano holes can have excellent resistance toany steps or uses where a stress is applied such as polishing andannealing, and their applications can be significantly broaden.

Also, according to the present invention, there can be stably producedthe anodically oxidized alumina nano holes that are penetrated to anelectrode of the under layer made of, for example, noble metal. Theenclosing material can be uniformly electrodeposited in the nano holes.Using such nano holes, magnetic mediums, quantum effect devices, opticaldevices and the like can be produced.

The present invention enables the anodically oxidized alumina nano holesto apply to various forms, and to significantly broaden theirapplications. The nano structure of the present invention can be used asthe functional material itself, and can also be used as an under layermaterial or a mold of a further novel nano structure.

As described above, the present invention can provide the nano structurein which the bottoms of the nano holes are penetrated to the under layerhaving excellent tightness between the anodically oxidized alumina nanohole layer and the conductive under layer. Also, the present inventioncan provide a method for easily producing the above-mentioned siliconnano structure.

While the present invention has been described with reference to whatare presently considered to be the preferred embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments. On the contrary, the invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims. The scope of the following claims is to beaccorded the broadest interpretation so as to encompass all suchmodifications and equivalent structures and functions.

1. A structure including: a substrate; a first layer including at leasta first hole; and an adhesive second layer disposed between thesubstrate and the first layer, wherein the second layer increasesadhesiveness between a surface of the substrate and the first layer, thesecond layer has a plurality of second holes, the first hole of thefirst layer communicates with the plurality of second holes of thesecond layer, and a magnetic material is filled into the first hole andthe plurality of second holes.
 2. A structure according to claim 1,wherein a third layer having conductivity is formed between thesubstrate and the second layer.
 3. A structure according to claim 1,wherein the first hole has a diameter of several nm to several hundrednm.
 4. A structure according to claim 3, wherein the second layercontains Si and Al.
 5. A structure according to claim 1, wherein 80% to100% of the atomic weight of the adhesive second layer is silicon, basedon the total atomic weight of all components of the adhesive secondlayer other than oxygen, and 0% to 20% of the atomic weight of theadhesive second layer is aluminum, based on the total atomic weight ofall components of the adhesive second layer other than oxygen.
 6. Astructure according to claim 1, wherein the adhesive second layerincreases the resistance of the first layer to mechanical andtemperature stresses.